1. Field of the Invention
The present invention relates to an optical system and an image pickup apparatus including the optical system. The present invention is applicable to an optical apparatus such as a silver-halide film camera, a digital still camera, a video camera, a telescope, binoculars, a projector, and a copy machine.
2. Description of the Related Art
In general, there is a demand for small optical systems having a small overall lens length (overall optical length, i.e., distance between a first lens surface on an object side and an image plane) for use in image-pickup apparatuses, such as digital cameras and video cameras.
As the overall size of the optical system is reduced, aberrations, in particular, chromatic aberrations including an axial chromatic aberration and a chromatic aberration of magnification are generally increased and the optical performance is degraded.
According to known methods for reducing the chromatic aberrations, an anomalous partial dispersion material is used as an optical material or a diffractive optical element is placed in an optical path.
In some telephoto type optical systems, the chromatic aberrations are corrected by placing a lens made of an anomalous partial dispersion material in a front lens unit, at which paraxial marginal rays and paraxial chief rays pass through positions distant from an optical axis. For example, the chromatic aberrations are corrected using a front lens unit including a lens with a positive refractive power composed of a low dispersion optical material, such as fluorite, having an anomalous partial dispersion and a lens with a negative refractive power composed of a high dispersion optical material. Examples of such telephoto type optical systems are described in U.S. Pat. Nos. 4,241,983, 4,348,084, and 6,115,188.
In other telephoto type optical systems, the chromatic aberrations are corrected without the use of the optical material having an anomalous partial dispersion by using a diffractive optical element. Examples of such telephoto type optical systems are described in U.S. Pat. Nos. 5,790,321 and 5,629,799. More specifically, U.S. Pat. Nos. 5,790,321 and 5,629,799 describe optical systems with an F-number of about 2.8 which relatively accurately correct the chromatic aberrations by combining a diffractive optical element and a refractive optical element.
In general, diffractive optical elements are characterized in that the absolute value of a numerical value corresponding to an Abbe number is small (around 3.45). In addition, simply by slightly changing the power (reciprocal of focal length) obtained by diffraction, the chromatic aberrations can be largely changed while hardly affecting a spherical aberration, a comma aberration, an astigmatism, etc.
A liquid material having a relatively high dispersion and a relatively anomalous partial dispersion is known as an optical material that provides a chromatic-aberration correcting function similar to that obtained by the optical characteristics of the diffractive optical elements. Achromatic optical systems using such a material are suggested in, for example, U.S. Pat. Nos. 5,731,907 and 5,638,215.
An optical system in which a mixture obtained by dispersing indium tin oxide (ITO) fine particles in a transparent medium is used as a material having anomalous partial dispersion characteristics to provide an achromatic function is also known (U.S. Pat. No. 7,057,831).
In addition, an optical system in which resin or mixture obtained by dispersing TiO2 fine particles in a transparent medium is used as a material having anomalous partial dispersion characteristics to provide an achromatic function is also known (U.S. Pat. No. 7,193,789).
With the telephoto type optical system using fluorite or the like as the optical material, the chromatic aberrations can be easily corrected if the overall lens length is set to be relatively long.
However, if the overall lens length is reduced, the chromatic aberrations are increased and cannot be corrected with high reliability. This is because the chromatic aberrations generated by the front lens with the positive refractive power are simply reduced by using the material like fluorite that has low dispersion and an anomalous partial dispersion. To correct the chromatic aberrations increased as the overall lens length is reduced, in the lens having a low dispersion glass such as fluorite with a large Abbe number, the refractive power of a lens surface must be largely changed to change the chromatic aberrations.
Therefore, it is difficult to achieve both the correction of the chromatic aberrations and the correction of other aberrations, such as the spherical aberration, the coma aberration, and the astigmatism, which are generated as the refractive power is increased.
The diffractive optical element provides a sufficient chromatic-aberration correcting function. However, the diffractive optical element generates unnecessary diffracted light with diffraction orders other than that of the diffracted light used in practice. The unnecessary diffracted light functions as colored flare light that degrades the imaging performance.
In order to reduce the unnecessary diffracted light, a so-called layered diffractive optical element in which a plurality of blaze diffraction gratings are laminated along an optical axis can be used. Thus, the energy can be concentrated at the designed diffraction order and unnecessary diffracted light can be greatly reduced. However, when high-luminance objects are shot, flare is still generated because of the unnecessary diffracted light.
As a method for manufacturing the diffractive optical element, a method of forming the diffractive optical element composed of an ultraviolet curable resin or the like by molding is known. However, according to this method, the sensitivity of diffraction efficiency of the diffractive optical element is extremely high. Therefore, a high-accuracy mold must be used and high molding accuracy is required. Thus, the diffractive optical element is difficult to manufacture.
With the method using the liquid material, a structure for sealing the liquid material is necessary. Therefore, in the case of using the liquid material as an optical material, it is difficult to manufacture the structure in which the liquid material is sealed. In addition, the characteristics including the refractive index, dispersion, etc., vary in accordance with the temperature variation and it is difficult to maintain a high environment resistance.
ITO, which is used as an optical material for correcting the chromatic aberrations, has a relatively low permeability compared to other optical materials.
Therefore, if ITO is used as the material having an anomalous partial dispersion for forming a lens or a layer having a refractive power in an optical system, the chromatic aberrations must be corrected without increasing the thickness of the lens or the layer in an optical axis direction.